As used herein and in the claims, the terms "electrical steel" and "silicon steel" relate to an alloy, the typical composition of which by weight percent falls within the following:
Carbon: 0.060% Maximum PA1 Silicon: 4% Maximum PA1 Sulfur and/or Selenium: 0.035% Maximum PA1 Manganese: 0.02% to 0.4% PA1 Aluminum: 1.0% Maximum PA1 Boron: 0.01% Maximum PA1 Iron: Balance PA1 AL.sup.+++ as Al.sub.2 O.sub.3 : 3-11% by weight PA1 Mg.sup.++ as MgO: 3-15% by weight PA1 H.sub.2 PO.sub.4.sup.- as H.sub.3 PO.sub.4 : 78-87% by weight PA1 Al.sup.+++ as Al.sub.2 O.sub.3 : 3-11% by weight PA1 Mg.sup.++ as MgO: 3-15% by weight PA1 H.sub.2 PO.sub.4.sup.- as H.sub.3 PO.sub.4 : 78-87% by weight
While the insulative coatings of the present invention are applicable to carbon steels for electrical uses, non-oriented silicon steels and silicon steels having various orientations, they will, for purposes of an exemplary showing, be described with respect to their application to cube-on-edge oriented silicon steel.
Such silicon steel is well known in the art and is characterized by the fact that the body-centered cubes making up the grains or crystals are oriented in a position designated (110) [001] in accordance with Miller's Indices. Cube-on-edge oriented sheet gauge silicon steel has many uses, an exemplary one of which is in the manufacture of laminated magnetic cores for power transformers and the like. In such an application, the magnetic characteristics of the cube-on-edge oriented silicon steel are important, and primary among these are core loss, interlaminar resistivity, space factor and magnetostriction.
It has long been recognized by prior art workers that the magnetic characteristics of cube-on-edge oriented silicon steel, and particularly those mentioned above, are enhanced if the silicon steel is provided with a surface film or glass which provides insulation to prevent wraps or laminations from "shorting" in a transformer. In the commercial manufacture of cube-on-edge oriented silicon steel, an annealing separator is used during the final anneal to which the silicon steel is subjected (i.e. that anneal during which the cube-on-edge orientation is achieved). When an appropriate annealing separator is used, as for example magnesia or magnesia-containing separators, a glass film is formed upon the surfaces of the silicon steel. This glass film is generally referred to in the industry as a "mill glass. "
In some applications it is desirable to have an applied insulative coating rather than, or in addition to, the mill glass formed during the high temperature, orientation-determining anneal. This has led to the development of phosphate coatings, magnesium phosphate based coatings and aluminum phosphate based coatings, all of which produce an applied insulative coating on electrical steel when appropriately dried and cured.
U.S. Pat. Nos. 3,948,786 and 3,996,073 teach, respectively, means and method for the provision of improved insulative, tension-imparting coatings for electrical steel, with or without a mill glass base coating. The teachings of these patents are incorporated herein by reference. Briefly, these patents teach that excellent insulative coatings can be formed on electrical steels by applying thereto an aluminum-magnesium-phosphate solution containing Al.sup.+++, Mg.sup.++ and H.sub.2 PO.sub.4.sup.- concentration in the following relative relationship on a water-free basis:
The total weight percentages of these components must be 100 on a water-free basis.
A colloidal silica (SiO.sub.2) solution may be added to the aluminum-magnesium-phosphate solution. If the concentration of Al.sup.+++, Mg.sup.++ and H.sub.2 PO.sub.4.sup.- (again calculated as Al.sub.2 O.sub.3, MgO and H.sub.3 PO.sub.4, respectively) constitutes 100 parts by weight on a water-free basis, the colloidal silica will comprise from 0 to 150 parts by weight on a water-free basis. When colloidal silica is present, the total weight percent of Al.sup.+++ (as Al.sub.2 O.sub.3), Mg.sup.++ (as MgO), H.sub.2 PO.sub.4.sup.- (as H.sub.3 PO.sub.4) and SiO.sub.2 must be 100 on a water-free basis. At least 45% by weight of the solution is water. To the solution may be added chromic anhydride (C.sub.r O.sub.3) to stabilize the solution prior to application to the strip, to improve solution wettability and to improve moisture resistance of the final coatings and interlamination resistivity after stress relief anneal. The coating solutions of these references may be applied to silicon steels (with or without a mill glass base coating) in any suitable and conventional manner. The coated silicon steels will thereafter be subjected to a heat treatment to dry the solution and form the desired insulative glass film thereon.
The present invention constitutes an improvement upon the teachings of the above mentioned U.S. Pat. Nos. 3,948,786 and 3,996,073. In the commercial practice of these patents, it has been common procedure to provide a colloidal silica solution content of 50 volume percent with respect to the aluminum-magnesium-phosphate solution, hereinafter for convenience referred to as the "premix" solution. Thus, it has been preferred to use a coating solution having a premix solution to colloidal silica solution ratio of 1:1, i.e. a coating solution comprising 1 part premix solution by volume and 1 part colloidal silica solution by volume.
The present invention is based upon the discovery that optimum magnetics of the coated electrical steel are achieved when the coating solution contains colloidal silica in an amount of from that amount which will still permit the formation of a good glass, up to 40 volume percent colloidal silica solution (i.e. up to a 1.5:1 premix solution to colloidal silica solution). To achieve this, additional Mg.sup.++ should be added to the solution up to that amount which will go into solution. To the premix solution should also be added from about 5 to about 20 parts boric acid (H.sub.3 BO.sub.3) per 100 parts of H.sub.3 PO.sub.4 on a water-free basis. Chromic anhydride (C.sub.r O.sub.3) is added to the premix to improve the wettability of the coating solution and to increase the moisture resistance of the resulting coating and interlaminar resistivity after stress relief anneal. The chromic anhydride addition also improves the appearance of the coating. It has further been found that with chromic anhydride present, more MgO will go into solution.
With the use of the improved coatings of the present invention, it has been found that better core loss values at inductions greater than 10 kg are achieved. Since colloidal silica is the most expensive ingredient of the coating solutions, reducing the volume percent of collodial silica will result in substantial cost savings. Furthermore, better coating adherence is achieved. The coatings of the present invention enable different top and bottom coating weights to be utilized, without adverse effects on magnetic quality. The coatings of the present invention are slightly rougher than those commercially achieved in accordance with the teachings of U.S. Pat. Nos. 3,996,073 and 3,948,786, which assists during the stacking of coated laminations in the manufacture of transformer cores and the like.
It has additionally been found that if the coating solutions of the present invention are diluted to form a uniform coating as thin as possible and having a coating weight of less than 2 grams per square meter on each side of the strip and preferably less than 1 gram per square meter on each side of the strip, they will, upon drying and curing, form excellent anti-stick coatings for non-oriented, semi-processed electrical steels.